This invention relates to nuclear reactor fuel element assemblies and to axial flow cavitation suppressing impedances therefor. The invention is concerned with the problem of cavitation arising from impedances inserted in coolant flow channels in order to regulate the pressure drop in the channels and with flow induced fatigue problems. Cavitation is particularly objectionable in fluid flow channels of a nuclear reactor because it creates `noise`, erosion and vibration of components. It is known to put impedances in the coolant flow channels of a nuclear reactor core to regulate the flow of coolant through fuel elements and thereby control the operating temperatures of the fuel. Early forms of impedance were of the orifice plate type but these gave rise to severe cavitation with large coolant flows and large pressure drops across the orifice. An improved impedance for fluid flow is disclosed in U.S. Pat No. 3,678,963 the impedance comprising a tubular body member housing a series of flow-defining plates arranged transverse to the longitudinal axis of the body member, the series being formed from a first and a second set of plates arranged so that a plate of one set alternates, as viewed in the direction of flow, with a plate of the other set, the first set of plates being of woven wire mesh form and there being a downstream space between plates of the first set to define a flow which is cavitation suppressed by virtue of eddy-induced pressure drops developed in the downstream spaces and the second set of plates being apertured to define the presented flow area of the impedance. An impedance of the improved kind will be referred to hereinafter as an axial flow cavitation suppressing impedance of the described kind. It is an object of the present invention to provide a nuclear reactor fuel element assembly incorporating an axial flow cavitation suppressing impedance generally of the described kind but which offers more resistance to vibration.